Trump fired off a letter Wednesday after Seinfeld canceled his planned appearance at a Sept. 13 event benefiting the St. Jude Children's Research Hospital for the Eric Trump Foundation, the New York Post reports. The comedian reportedly backed out in response to Trump's questioning of President Barack Obama's citizenship.

"I just learned you canceled a show for my son's charity," Trump wrote, "because of the fact that I am being very aggressive with respect to President Obama, who is doing an absolutely terrible job as our leader. We don't care that you broke your commitment even though the children of St. Jude are very disappointed, and despite the fact that your manager clearly stated you are 'truly a man of his word.'"

The Donald then began taking jabs at The Marriage Ref, Seinfeld's NBC relationship comedy show that premiered last year.

"What I do feel badly about is that I agreed to do, and did, your failed show, The Marriage Ref, even though I thought it was absolutely terrible," Trump wrote. "Despite its poor ratings, I didn't cancel on you like you canceled on my son and St. Jude. I only wish I did. You should be ashamed of yourself!"

Although Seinfeld backed out because he felt Trump's "kind of demagoguery has no place in public discourse," he made a contribution to the Eric Trump Foundation and St. Jude's.